Saturday, January 7, 2012

Projections

Well, I said I was going to look at Trackleaders's projections (scroll down to "Leaderboard," click "Refresh with projections") during the Knik 200, and I just did. I have to admit that I'm not really sure what's going on, but Matthew Lee from Trackleaders has been wonderfully responsive to questions and I hope he can help me out here. I hope that explaining what I find confusing might help provide some insight into how to look at these particular data. There are two issues I've noticed:

Here's the easy one, for starters: I'm looking at the projections for arrivals into Skwentna. Zoya Denure is marked on the leaderboard as having arrived, but Vern Halter is not, Christine Roalofs is not, and other folks who've arrived (you can see it if you eyeball the tracker - there's a clump of them there near Zoya, they're not moving, and if you look at individual pages they've been sitting there for some time) have not. If you look at the tracker what appears to have happened is that the checkpoint is marked on the map as being further down the trail than it actually is, or at least the dog lot is not as far down the trail as the checkpoint marker. Zoya is the closest to the checkpoint marker, so I'm guessing that she's within some margin that allows the software to recognize her as having arrived, while everybody else whose parked is just a hair too far and the software's not marking them as having arrived even though they have. What that means is that it's calculating projected arrival times despite them already being there. So, sometimes you have to recognize that some data aren't going to help you much and throw it out. As I've described in other places, if the raw data (in this case, GPS breadcrumbs) and the summaries of the data are telling you different things, trust the raw data.

But let's take a look at someone who's through Yentna and on the trail towards Skwentna: Jeremy Rutledge. Here's how he shows up in the projections:

This was taken when Jeremy was 25.9 miles from Lance Mackey, who we know is in Skwentna. If you click on this so that you can see the times, Jeremy and many of the others are projected as arriving in Skwentna before they left Yentna. The laws of physics say this isn't possible and I usually trust the laws of physics, so the inevitable question is what's going on here that's incorrect. Chris glanced at it, said "Oh, right, this is an out-and-back trail!" If you've read this piece on what data are actually included in each message from the Spot you might remember that it's basically just a geographic coordinate and not much else. Not speed, not direction. So our best guess is that the projections are being calculated based on an incorrect assumption about the direction the team is traveling at this spot on the trail - i.e. the calculations might be based on arithmetic in which it's assumed the team is heading south, when it's really heading north. We don't know that's the case but it's our best guess at the moment.